1. Field of the Invention
The present invention relates to a method and a node for maintaining a multimedia session in an alive state for a mobile station following a release of a network attachment resulting from a handoff from a first Packet Data Node to a second Packet Data Node and following dormancy state of the mobile station.
2. Description of the Related Art
IP Multimedia Subsystem (IMS) supports multimedia sessions between an end-user terminal, for instance a Mobile Station (MS), and IP (Internet Protocol) multimedia servers and content providers. A generic name for an IP multimedia server is an Application Function (AF). The AF offers applications that require the control of IP bearer resources. An example of a service supported by the IMS may be illustrated by a discussion of SIP (Session Initiation Protocol), which is a signaling protocol designed for support of telephony, event notifications, interactive gaming, instant messaging and other interactive communication sessions between users over Internet by use of multimedia sessions. SIP is described in RFC 2543 of IETF (Internet Engineering Task Force). SIP messages between the MS and a network are carried over a multimedia session. The multimedia session is called SIP session when SIP is used as a call control mechanism. A popular mobile application of SIP is named PTT (Push To Talk). PTT uses SIP as the call control mechanism. PTT carries Voice over Internet Protocol (VoIP) by establishing a VoIP connection over a multimedia session. A mobile user can initiate a SIP session enabling him to communicate with peers by use of a PTT call. To talk to or listen to peers once the SIP session has been established, the user simply needs to press on a single PTT button on his MS. By pressing this button, the user makes his MS connect to a Radio Access Network (RAN), establish a packet data session with a Packet Data Node (PDN), and send a SIP message over the SIP session to an AF, also named application server, which is in control of the PTT call. When the user no longer needs to talk to or listen to his peers, he may release his communication by releasing the button, without releasing the SIP session. While there is no active communication, the radio link is released, normally following a few minutes of inactivity. The user may resume communication with his peers, maybe several hours later, by simply pressing again on the same button. The SIP session, hence the multimedia session between the MS and the application server, may have remained alive for a silent period lasting for several hours. It is by maintaining the multimedia session that a VoIP connection may be resumed almost instantaneously when the user desires to communicate again.
CDMA2000 systems, as defined by 3GPP2 (3rd Generation Partnership Project 2), support circuit switched connections as well as packet data connections. To set up a circuit switched connection, the CDMA2000 system sets up: (i) a radio link between a Mobile Station (MS) and a Radio Access Network (RAN) and (ii) a terrestrial link between the RAN and a Mobile Switching Center (MSC). In a circuit switched connection, a call release implies releasing all resources allocated for serving the call. A packet data connection, also named packet data session, is characterized by setting up of a radio link between the MS and the RAN and by setting up of a network attachment comprising a Point to Point (PPP) connection between the MS and a Packet Data Node (PDN), named Packet Data Service Node (PDSN) in CDMA2000 systems, connected to the RAN, also by use of a terrestrial link. The PDSN assigns an IP address to the MS. In cases where the packet data session is established for the purpose of using a multimedia service, a multimedia session may further be established between the MS and an Application Function. In a packet data session, when the MS is neither sending nor receiving data and has been inactive for a certain period, the radio link between the MS and the RAN is released. Despite this release of the radio connection, the network attachment comprising the PPP connection between the MS and the PDSN and the IP address assigned to the MS are still maintained. This state is called dormancy. By maintaining the network attachment, the PPP connection and the IP address while the MS is in the dormancy state, the multimedia session is maintained in an alive state. Hence, a service may be reactivated quickly when there is data to be sent either to or from the MS.
General Packet Radio Service (GPRS) systems, as defined by 3GPP (3rd Generation Partnership Project), are very similar to the CDMA 2000 systems mentioned hereinabove in that they also support packet data sessions by setting up a radio link between a MS and a RAN, and further setting up a network attachment comprising an IP connection between the MS and a PDN, named GGSN (Gateway GPRS Serving Node) in GPRS systems. Like the PDSN in a CDMA 2000 system, the GGSN assigns an IP address to the MS.
FIG. 1 shows an exemplary IMS architecture 100 for interworking between a Application Function (AF) 110, a PDN 120 and a PDF (Policy Decision Function) 130, serving a MS 140 in an IP Multimedia Subsystem in accordance with the Prior Art. While for clarity reasons, a single AF, PDF and PDN are shown on FIG. 1, a typical IMS architecture would most probably include a number of PDNs, PDFs and AFs. A network attachment 160 is shown between the PDN 120 and the MS 140. The network attachment 160 is set up transparently over a RAN (not shown). Those knowledgeable with the IP Multimedia Subsystem will recognize that a P-CSCF (Proxy Call Session Control Function) is a well-known example of possible Application Function (AF) types.
The PDF 130 role in the IMS architecture is to coordinate activities between a bearer plane and a control plane in a MMD (MultiMedia Domain). The bearer plane transports user data between network entities involved in a call. Examples of such network entities comprise the MS 140, the PDN 120 and Internet. The control plane supports call control such as SIP messages, as well as other applications and services. An example of call control message is the aforementioned SIP message initiated by the MS 140 when the user presses a PTT button on the MS. An example of a policy enforced by the PDF comprises translating of a service request parameter sent from the MS 140 as a result of a PTT session initiation into a Quality of Service requirement. The AF 110 maintains in a database a context of the multimedia session, comprising for example an IP address of the MS 140, filters and header compression information.
As shown on FIG. 1, the PDF 130 interfaces with the PDN 120 on one hand via a Go interface 140, and with the AF 110 on the other hand via a Gq interface 150. The Go interface 140 is described in 3GPP TS 29.207 V6.2.0 (2004-12). The Gq interface 150 is described in 3GPP TS29.209 V6.1.0 (2004-12). Events taking place at the session control plane, in the AF 110, are reported to the PDN 20, via the PDF 130, for authorization and enforcement. Likewise, events at the bearer plane, in the PDN 20, are reported to the session control plane via the PDF 130, so that they can be properly acted upon, in a manner that depends on the event type.
One of the events reported to the PDF by the PDN is when an underlying bearer is released, for example when the network attachment 160 between the MS and the PDN is torn down, also resulting in a release of the PPP connection and of the IP address assigned to the MS. The PDF in turn reports that event to the AF. Currently, the AF is required to clear the multimedia session, for example a SIP session, including its context, should such an event be reported. This is done to ensure that the events that both the control and the bearer plane report separately do match, as a means to also ensure consistency in charging information for the multimedia session in place.
When performing a fast handoff between PDNs, the MS moves from a first PDN to a second PDN using procedures defined as fast handoff in X.S0011 and IOS 4.3 specifications of the 3GPP2. In fast handoff, the MS maintains the network attachment 160 comprising the PPP connection and a current packet data session anchored in the first PDN until the MS goes dormant or the MS disconnects. During that period before the dormancy state, as the MS is connected to the second PDN, this second PDN provides a tunnel between the MS and the first PDN. If for example a PTT call is in an active state, implying that the user is actively speaking to or listening to a peer, the multimedia session comprising the VoIP connection is maintained over the PPP connection between the MS and the first PDN, until the end of voice activity, when the MS enters the dormancy state.
Considering now a traffic case where a mobile hands off from a first PDN to a second PDN. In this traffic case, clearing of IMS sessions occur when the network attachment comprising the PPP connection is released and the IP address is cleared, at the time when the MS becomes dormant. This clearing of IMS sessions overlooks legitimate traffic cases where it would be advantageous to maintain call connections. Specifically in the case of VoIP session, when there is a handoff from one SIP domain to another SIP domain, a call is not allowed to continue. There is no transfer of call information between the first PDF and the second PDF. Currently, if the X.S0011 and IOS 4.3 specifications are followed, the call is cleared. For example in the case of a PTT session, the user will have to re-establish a new packet data session and a new multimedia session with a new IP address before resuming communication with his peers. This re-establishment of the new multimedia session implies more actions on the part of the user than simply pressing on one PTT button. This situation renders a service such as Push To Talk much less efficient and much less enjoyable for the user.
U.S. Pat. No. 6,654,359 issued in the names of La Porta et al. on Nov. 25, 2003 (hereinafter called La Porta) enables a MS handoff from a first, home or foreign network, to a second foreign network. La Porta provides a care-of-address to the MS, for tunneling purposes, so that it can continue using the same IP address without disruption. Further handoffs are transparent to the home network since the care-of-address does not change within the foreign network.
Canadian patent application number 2 359 040 published in the names of Hasan and Tiburtius on Sep. 27, 2001 (hereinafter called Hasan) describes a PPP connection established between a PDN and a mobile terminal, based on the received PDN information. The PPP connection is extended from the PDN to another PDN and the mobile node is informed that the handover is complete. The method of this invention requires setting up a second PPP connection with the second PDN. Hasan is only concerned with the handling of PPP connections between the access network and the PDN. It does not relate to the impacts of handoffs between the PDN and the rest of the network.
UK patent application number GB 2 278 090 published in the names of Cheung and Barrett on Sep. 27, 2001 (hereinafter called Cheung) provides a method for dynamic sharing of session management information upon handoff from a second generation to a third generation mobile network. Information is shared between GPRS Service Nodes (GSN) or between Mobile Switching Centers/Visitor Location Registers (MSC/VLR). Information is exchanged directly between the two systems, without having to involve a HLR (Home Location Register) until the end of the handoff process, in order to speed up the handoff. The disclosure makes no mention of a means or method to maintain a multimedia connection upon handoff. Specifically, Cheung does not propose any solution to the problem of maintaining a proper match between control plane and bearer plane events.
There would be a clear advantage of maintaining a multimedia session in an alive state with its full context, for a mobile station, when a network attachment is released following a handoff between packet data nodes, later followed by a dormancy state.